Motor control circuit



April 12, 1,949. c. M. GILBERT 2,466,984

MOTOR CONTROL CIRCUIT Filed Sept. 9, 1947 Zhwentor CLARKE M. GlLBERT Ottomeg Patented Apr. 12, 1949 MOTOR CONTROL CIRCUIT Clarke M. Gilbert, Chappaqua, N. Y., assignor to General Precision Laboratory Incorporated, a corporation of New York Application September 9, 1947, Serial No. 773,062

Claims. 1

This invention relates to a motor control circuit in which a small signal controls the speed and direction of rotation of a motor. More particularly the present invention relates to a motor control circuit in which a direct current series field motor is controlled in speed and direction of rotation by an alternating current signal and additionally receives its power from an alternating current source.

Heretofore, direct current series field motors have not been well adapted for servo control purposes because of the difiiculty in providing a simple circuit arrangement in which the direction of current flowing through a single field coil could be reversed by a change in sense or phase of a control or signal voltage.

Where the use of such a motor has been attempted in the past, comparatively complicated circuits have been necessary involving the use of numerous amplifier stages, mechanical switching circuits and the like.

In accordance with this invention, however, there has been devised a very simple circuit for obtaining reversal of direction of current fiow in the single field coil thereby to control the speed and direction of the motor. Additionally the invention incorporates means so that the entire circuit including the motor may be operated from alternating current mains without necessitating the addition of rectifying or other current conversion equipment.

Simplicity of circuit design and operation is achieved by the use of but two discharge tubes which are caused to perform the double functlon of acting as the rectifiers which convert the alternating current supply to direct current for energizing the motor and at the same time as the control mechanism which reverses the direction of rotation'of the motor in response to an impressed signal.

This advantageous result is accomplished by a new and novel circuit in which a pair of discharge devices are connected so as to operate as a full wave rectifier for converting the alternating current of the supply source to the direct current necessary as the energizing means for the motor and at the same time operate as differential amplifying means to produce a field current such that the resultant flux induced in the field coil may be reversed in direction depending on the phase of the impressed control signal.

The exact nature of the invention will be more clearly understood from the following detailed description taken together with the attached drawings, in which:

Figure 1 is a schematic diagram of one circuit of the invention; and

Figure 2 is a modification of the circuit of Fig. 1. In the two figures corresponding reference numerals have been used to denote corresponding elements, thereby facilitating comparison of the circuits depicted in these figures.

Referring now to Fig. l a pair of discharge tubes l8 and I9 have their anodes l8 and I1 connected to the opposite ends of the secondary l4 of transformer the primary l2 of which is energized by a source of alternating current indicated diagrammatically at I3. Cathodes 2| and 22 of tubes l8 and i5 are each connected to ground through resistors 23 and 24, each of which is bypassed by condensers 26 and Ti. The anode circuits of each of the tubes H3 and I9 are completed by a common circuit consisting of the conductor 29 connected to a center tap 28 on the secondary H, the armature 3 I, conductor 30 and ground, the armature 3| being shunted by condenser 32.

Tubes I8 and i9 thus constitute a full wave rectifier circuit for the armature 3| converting the alternating current potential eggisting across secondary M to a pulsating unidirectional current which is filtered and smoothed to .a direct current by the filtering action of the armature 3| and shunt condenser 32. i

The grids 33 and 34 of tubes I8 and I9 are connected together and to a source of alternating current control signal which has here been indicated for purposes of simplicity as a potentiometer 36 grounded at its mid-point 31 and having a movable contact 38 to which the grids 33 and 34 are connected. Alternating current is supplied to the potentiometer by leads 39 and 4| connected to the alternating current source [3. I

It is to be understood of course, that the alternating current control signal may be derived from any of the usual sources and by any of the usual means employed in servo control systems, the particular source depicted in the drawings being by way of illustration only and is not to be considered as limited thereto.

Direct current bias is obtained for tubes l8 and I9 by the potential drop in resistors 23 and 24 respectively connected in the cathode circuits of these tubes and bypassed by condensers 26 and 21. The upper, or cathode ends, of these resistors are connected to opposite ends of the field coil 42 through leads 43 and 44 so that the potential impressed across the field coil 42, if any, is the relative difference in potential of the cathodes 2| and 22.

potentiometer II. Under these areas The operation of the circuit disclosed in Fig. 1 is as follows:

Assume flrst that no control signal is impressed on the grids 83 and 84. That is, that the movable contact a has been adjusted at the grounded mid-point". The anodes it and ll of tubes is and u by reason of their connection to opposite ends of the secondary ll of transformer II are alternately made positive on successive half cycles of the alternating supply source and plate current is caused to flow alternately in each of tubes I I and I. Since the plate current of each tube includes the armature II a full wave rectifled current is produced therein by the alternate operation of tubes II and II.

The plate current flowing through tube ll produces a potential drop in resistor 23 connected in the cathode circuit thereof and similarly plate current flowing through tube It produces a potential drop in resistor 24. Under the assumption that no control signal is impressed on the grids l3 and 34, the plate currents in tubes II and I! are equal producing equal potential drops in resistors 23 and 2 and hence cathodes 2| and 22 are at equal potentials above ground. No potential diiference, therefore, is impressed across field coil 42 by the connections 43 and 44, no current flowzetherethrough and the armature 3i does not rota Assume now a condition where an alternating current signal is applied to the grids of the tubes I. and II, for example, a condition as represented by the diagram of Fig. 1 where the movable contact II is somewhat above the mid-point ll of circumstances the grids a and II will have impressed thereon an alternating current potential of the same frequency as that of the supply source I: and the signal applied to the grid of one tube will be in phase with its plate potential while that applied to the grid of the other tube will be out of phase with its plate potential. For example, in the circuit as depicted in Fig. 1, when movable contact II is positive with respect to mid-point 31 and hence both grids 33 and 34 are driven in a positive direction, anode I1 is positive while anode I6. is

- negative, there being a phase inversion from priman? 12 to secondary H of transformer i I. On the'other hand when at the next half cycle anode ll becomes positive so that tube ll may draw current the grids are driven in a negative direction. The net result is that the plate current through tube it increases while that through tube It decreases. There is then an increased potential drop in resistor 24 in the cathode circuit of tube II and a decreased potential .drop in resistor 23 in the cathode circuit of tube ll. Cathode 22, therefore, assumes a higher potential than cathode II and since a diiference of potential exists across the field coil 42 current flows therein resulting in rotation of armature Si in a sense dependent on the direction of current flow in the fleld coil.

It will be apparent from the above that when the phase of the control signal is reversed by movement of the contact 38 to a point below the mid-point 31, a reversal of the above action occurs in which the plate current through tube It increases while that through tube l9 decreases causing the potential impressed across fleld coil 42 to be reversed and hence the flow of current therein and the direction of rotation of armature II to be reversed.

The circuit of Fig. 2 is similar to that of 1 except cathode ii is connected to one end of fleld coil ll through self-biasing resistor l2 and cathode 22 is connected to the other end of fleld coil ll through self-biasing the fleld coil II is grounded at its mid-point by conductor II.

In this circuit the plate current of tube ll flows through the upper half of field coil ll while that of tube It flows through the lower half. As long as no alternating current signal is impressed on these tubes the currents through the two halves of the neld coil are equal and opposite and there is no resultant flux produced which would cause the armature II to revolve.

asignalisappliedtogrldsll and 84 which results in unequal plate currents in the tubes in the manner heretofore described, the currents in the two halves of the fleld coil become unequal and there is a resultant flux causing the armature to revolve in one direction or the other depending on the direction of the flux.

There is thus provided a'control circuit in which a single pair of tubes both act as a rectifler circuit for supp y ng direct current to a motor armature and as amplifying means for controlling the direction and speed of the motor in accordance with the phase and amplitude of a control signal.

What is claimed is:

1. A motor control circuit comprising a pair of tubes, each having at least anode, cathode and grid electrodes, an alternating current supply source connected to energize said tubes in phase opposition, a motor having a single fleld coil, means connecting the motor armature in the anode circuit of each of said tubes for energizing said armature by the rectified current flowing in said anode circuits, a circuit simultaneously supplying an alternating current control signal to the grid electrodes of each of said tubes and circuit connections from said cathodes to opposite ends of said field coil.

2. A motor control circuit according to claim 1 in which self-biasing resistors are connected in the cathode circuits of each of said tubes between the opposite ends of said field coil and said motor armature. 1

3. A motor control circuit according to claim 1 in which self-biasing resistors are connected in the circuit connecting said cathodes to opposite ends of said field coil and the midpoint of said field coil is connected to said motor armature.

4. A motor control circuit comprising a source of alternating current, a full wave rectifier circuit connected to said source for converting said alternating current to a unidirectional current, said full wave rectifier circuit includinga pair of tubes each having at least an anode, cathode and control grid, a motor having an armature and a single field coil, a circuit for energizing said armature by said unidirectional current, a first electrical connection between one of said cathodes and one end of said fleld coil, a second electrical connection between the other of said cathodes and the opposite end of said field coil, and a circuit for simultaneously supplying an alternating control signal to each of said control grids.

5. a motor control circuit according to claim 4 in which each of said first and second electrical connections are connected to said motor arrnature through a. resistor.

6. A motor control circuit according to claim 4 in which each of said first and second electrical connections includes a series resistor and the midby the present invention I point oi said iield coil is connected to said motor armature.

7. A motor control circuit comprising an alternating current supply source, a pair of tubes each having at least an anode, cathode and control grid, a circuit energizing said anodes by said supply source in substantial phase opposition, a motor having a field coil and an armature, a circuit connecting the opposite ends oi said field coil to the respective cathodes of said tubes, an alternating current signal circuit simultaneously supplying the control grids of each of said tubes with an alternating current potential of the same amplitude and phase and a circuit for supplying said motor armature with a unidirectional current.

8. A motor control circuit comprising, an alternating current supply source, a pair of tubes each having at least an anode, cathode and control,

grid, a circuit energizing said anodw by said alternating current source in substantial phase opposition, a motor having a field coil and an armature, a circuit connecting the opposite ends of said field coil to the respective cathodes of said tubes, an alternating current signal circuit simultaneously supplying the control grids of each of said tubes with an alternating current potential of the same amplitude and phase, and a circuit,

including said motor armature in common with the cathode-anode circuits 0! each 0! said tubes. A motor control circuit according to claim 8 in which self-biasing resistors are connected in,

the cathode circuits of each of said tubes between their junctures with the opposite ends of the ilelclv coil and the common cathode-anode circuit.

1 A motor control circuit according to claim 8 in which self-biasing resistors are connected.

between each of said cathodes and the respective ends of. said field coil and the midpoint of said.

REFERENCES crrsn The following references are of record inthe;

file of this patent:

UNITED STATES PATENTS Number Name 1,655,036 Alexanderson et a1. Jan. 3, 1928 1,936,407 Palmer Nov. 21, 1933, 2,150,265 Conover Mar. 14, .1939

FOREIGN PATENTS Number Country Date 750,213 France May. 22, 1933; 

